This project aims to identify some of the molecular and cellular determinants involved in murine leukemia virus (MuLV) infectivity and pathogenesis by the establishment of in vitro systems for host range sub- type characterization and lymphoid cell transformation. We have demonstrated by site-directed mutagenesis that a single amino acid change in the envelope protein of an ecotropic MuLV can radically alter the infective host range, i.e. resulting in the ability to infect a mouse cell line restrictive to the parental MuLV while greatly diminishing infectiousness for mouse cell lines permissive to the parent virus. This alteration in host range sub-type is independent of host cell functions, although the magnitude of restriction of infection can be influenced by host cell properties. The effects of this single amino acid change, and a series of similar alteration in the same envelope protein structural element, on receptor binding and utilization are currently being tested. In addition, in order to develop a model system for the lymphomagenicity of MuLV in vivo, we have established a cell culture system which is apparently permissive for MuLV-mediated lymphoid cell transformation. Normal mouse neonate thymocytes can be induced to persist and expand in culture supported by MuLV-infected and expressing thymic stromal cell lines; this capacity is greatly augmented by mink cell focus-inducing (MCF) viruses, MuLV known to accelerate lymphomagenesis in vivo. We are exploiting this system to dissect the viral and cellular determinants of susceptibility to retroviral transformation.